Impedance matching circuit for driving a speaker system

ABSTRACT

An impedance matched resonant circuit uses inductors coupling an audio source and a speaker array in order to reduce feedback reflection induced by an audio signal traveling from the audio source to the array of speakers. An RC circuit could also be coupled to the positive and negative terminals of the speaker array to reduce the slope differential of the audio signal at certain frequencies. The circuit can be packaged together in a single module with switches to activate and deactivate portions of the circuit to alter the effectiveness of the circuit depending upon need.

This application claims the benefit of U.S. Provisional Application No.61/823,737, filed May 15, 2013. This and all other referenced extrinsicmaterials are incorporated herein by reference in their entirety. Wherea definition or use of a term in a reference that is incorporated byreference is inconsistent or contrary to the definition of that termprovided herein, the definition of that term provided herein is deemedto be controlling.

FIELD OF THE INVENTION

The field of the invention is circuits to achieve substantiallyreflectionless impedance matching for use with a speaker system.

BACKGROUND

The background description includes information that may be useful inunderstanding the present invention. It is not an admission that any ofthe information provided herein is prior art or relevant to thepresently claimed invention, or that any publication specifically orimplicitly referenced is prior art.

All publications herein are incorporated by reference to the same extentas if each individual publication or patent application werespecifically and individually indicated to be incorporated by reference.Where a definition or use of a term in an incorporated reference isinconsistent or contrary to the definition of that term provided herein,the definition of that term provided herein applies and the definitionof that term in the reference does not apply.

Efforts have been made in the past to match the impedance of an incomingsignal to an output load. For example, U.S. Pat. No. 8,254,601 toFeldstein, issued Aug. 28, 2012, entitled “Impedance Matching SpeakerWire System” teaches a compensation circuit connected in series with aspeaker system and an incoming signal. More specifically, Feldsteindescribes a coaxial line having a predetermined characteristic impedancewherein the circuit is configured with circuit components that allow foradjustment of the impedance of the output circuit. Moreover, inFeldstein, the range of frequencies the tuning circuit can process islimited due to its predetermined characteristic impedance resulting ininefficient filtering of some frequencies. As a consequence of thesolution presented in Feldstein, the fidelity and level of definition inthe music is significantly impaired.

Additional efforts have been made through the use of capacitors. U.S.Patent Publ. 2007/0189554 to Innis, published Aug. 16, 2007, entitled“Audio speaker including impedance matching circuit” teaches impedancematching through integrated circuitry. By configuring the capacitors inparallel, a user can engage a switch to selectively shunt electricalcurrent around the capacitors. The result is altering of effectivespeaker impedance. By quantifying the value of the speakers, thematching circuit in Innis is limited to only matching certain predefinedinput values. Consequently, the efficiency of the matching circuit isgenerally not optimal.

Other efforts have been made to achieve reflectionless impedancematching. U.S. Pat. No. 1,832,452 to Feldtkeller et al. teaches atelephone interconnecting circuit that, by proportioning resistances toa transmission line, allows for termination that is practically freefrom reflection and attenuation from a transmitter to loud speakers.This solution, however, fails to take into account that audiosignals—unlike transmission line signals—can contain many differentfrequencies, and thus using a resistor (a static component) to matchimpedance is suboptimal over a range of frequencies.

U.S. Pat. No. 7,747,228 to Kasha et al. similarly shows efforts made toreduce signal reflection via impedance matching. Kasha et al. teachesthe use of an impedance matching component placed between an amplifieroutput and an antenna. The result is that signal reflections are eitherreduced or eliminated. This reference, however, does not provide anydetail as to what type of component might be used to achieve the goal ofreflectionless impedance matching.

U.S. Patent Application No. 2009/0175378 to Bogdan et al. teaches asystem capable of matching impedances over a wide range of radiofrequencies or other high frequency ranges. To accomplish this, itdigitally transforms a first load impedance into a second loadimpedance. However, this reference fails to teach impedance matchingusing passive components.

U.S. Application No. 2013/0325149 to Manssen et al. teaches a circuitthat can tune impedance to match a frequency coming into an antenna.However, this reference requires active controlling to create matchedimpedance and thus fails to appreciate that impedance matching can beachieved using passive components.

U.S. Pat. No. 8,190,109 to Ali et al. describes a system of activeimpedance matching. It measures the amount of reflected energy and then,using that value, varies the impedance of a component in the systembefore making another measurement and making another adjustment. Inother words, Ali et al. teaches a closed-loop control system forimpedance matching. In this way, it can converge on a solution resultingin substantially reflectionless matching. This system, as with Manssenet al., fails to appreciate that substantially reflectionless impedancematching can be achieved without the use of a feedback loop.

U.S. Pat. No. 8,472,907 to Yamagajo et al. teaches an antenna systemthat includes one or more impedance matching elements. However, devicesof this reference fail to teach impedance matching over a range offrequencies.

Other related references are similarly deficient. See U.S. PatentApplication No. 2010/0081379, U.S. Patent Application No.2011/0159832A1, U.S. Pat. No. 4,006,315, U.S. Patent Application No.2007/0201707.

Thus, there is still a need to further improve the power efficiency of aspeaker circuit by providing substantially reflectionless impedancematching using passive circuit components that allow for impedancematching over a wide range of frequencies.

SUMMARY OF THE INVENTION

As used herein, an “array of speakers” has a positive terminal, anegative terminal, and has one or more speakers coupled in paralleland/or serially with those terminals. In one aspect of the inventivesubject matter, a circuit for improving efficiency of an array ofspeakers such that the circuit, array, and audio signal source togethercomprise an impedance matched resonant circuit.

In preferred embodiments, the circuit includes (1) an inductor betweenan audio signal source and either positive or negative terminal of thearray, and (2) a RC circuitry coupled to the positive and negativeterminals of the array. In especially preferred embodiments, a firstinductor is operably disposed between the positive terminal of the arrayand the signal source, and a second inductor is operably disposedbetween the negative terminal of the array and the signal source.Additional circuitries could be coupled to the array of speakers toimprove the efficiency without departing from the scope of theinvention.

The inductors coupled to the positive and/or negative terminals made atleast in part with one or more magnetically responsive materials, as forexample paramagnetic materials, diamagnetic materials, and/orferromagnetic materials. Generally the magnetically responsivematerial(s) would comprise the core and/or gap of the inductor(s). Thecore and gap can be formed into a cylinder or bar with wire wrappedaround it, or in any other suitable arrangement including for example, atoroid with the coil of wire threaded through the central hole. In someembodiments, the inductor comprises a combination of materials, such astwo ferromagnetic, diamagnetic, and/or paramagnetic bars separated by aferromagnetic, diamagnetic, and/or paramagnetic material in a gapportion between the bars or a ferromagnetic, diamagnetic, and/orparamagnetic toroid having a ferromagnetic, diamagnetic, and/orparamagnetic gap.

The resistor in the RC circuitry can comprise a variable resistor, andthe capacitor in the second circuitry could comprise a variablecapacitor, each or both of which can be used to tune properties of anaudio signal. Contemplated variable resistors include, for example, apotentiometer, an electrically variable digital resistor, or a computerprogrammable logic controller. Contemplated variable capacitors include,for example, a rotary capacitor, a variable capacitance diode, a MEMSdigital capacitor, a BST digital capacitor, or a SOI/SOS digitalcapacitor. A switch or other user interface could be used to vary thecapacitance and/or resistance of the variable capacitor and resistor,respectively, to tune the circuit. Circuitry with a low capacitance anda high resistance will ensure that the audio signal has a larger slopedifferential near the drop off frequency, while circuitry with a highcapacitance and a low resistance will ensure that the audio signal has asmaller slope differential near the drop off frequency.

Any circuitry coupled to the audio speakers can include switches—eithertogether, in the alternative, or in any combination thereof. Theswitches are positioned such that when a switch is closed, the circuitrythat the switch it associated with is shorted out of the overallcircuit, effectively activating or deactivating a portion of thecircuitry. For example, a switch could be in parallel with an inductor,or in series with the RC circuitry, such that when the switch isactivated the inductor is effectively shorted from the circuit. Or aswitch could be in series with the RC circuitry, such that when theswitch is turned off, the RC circuitry is broken.

Where two inductors are used, a first circuitry has a first inductorcoupled to a positive terminal of an array of speakers configured toreduce reflection of an audio signal by the array of speakers to theaudio signal source, a second circuitry has a second inductor coupled toa negative terminal of the array of speakers configured to reducereflection of a signal from the negative terminal back to the audiosignal source and configured to provide symmetry in the circuit in termsof compensating for the electromagnetic reactions of the array ofspeakers, and a third circuitry has a resistor and a capacitor coupledto both the positive and negative terminals of the array of speakersconfigured to reduce a slope differential of the audio signal. In someembodiments, the resistor is a variable resistor, which can further beelectronically or mechanically variable. As discussed above, thecapacitor can be fixed or variable.

An audio enhancement kit is preferably configured as a box havingcircuitry within, and only a few exposed connection ports and controlports. Such a box has a positive and negative input terminal to coupleto an audio signal source, a positive and negative output terminal tocouple to the array, and control nodes to control which circuitries areactive, and the amount of inductance, resistance, and capacitance of thevarious inductors, resistors, and capacitors, respectively, wherevariable inductors, resistors, and capacitors are used. The controlswitches could be manual switches, but are preferably controlledelectronically so that a user could dynamically alter the properties ofthe audio enhancement kit from a single control station. The positiveand negative input and output terminals could be combined as a singleaudio port having positive and negative portions.

The inductance of each inductor determines beyond what frequencyamplitude drop-off will occur. For this reason, inductors can beselected to create signal drop-off at a desired frequency. Inembodiments where a user might prefer a drop off in signal levels beyonda first frequency at one time, and a drop off in signal levels beyond asecond frequency at a second time, a plurality of inductors could beprovided having switches that activate preferred inductors anddeactivate non-preferred inductors. Alternatively, a variable inductorcould be provided that is configured to vary inductance of the inductorcoupled to the positive terminal and/or the negative terminal, allowinga user to tune the circuit to produce a preferred sound quality.Switches could be provided that bypass the inductor altogether inembodiments where a user might not want the input signal to be alteredby an inductor at all. A single control station could, for example, setthe inductance in a first circuit to be 100 H during the first 30seconds of a song, raise the inductance to be 500 H for the next 30seconds, and then lower the inductance to 300 H for the rest of thesong. In this manner, a user could configure an audio enhancement kit tohave a plurality of signal drop-off levels throughout a song from asingle control station, where that song has a range of frequencies.

In embodiments where there is an inductor coupled to the negativeterminal of the array of speakers, the inductor is configured to provideimproved impedance matching. By matching the impedance between thenegative terminal of the array of speakers and the negative terminal ofthe audio signal source, the array of speakers is able to performbetter, because the driver of any speaker in the array of speakers wouldbe able to more freely resonate. This effect is brought about by theinductor's tendency to absorb current generated by the electromagneticeffect of the permanent magnet of a speaker moving relative to a voicecoil. Thus, coupling inductors to both the positive and negativeterminals of the array of speakers provides the benefit of compensatingfor electromagnetic effects on both the positive and negative sides ofthe array of speakers. In some embodiments, it is advantageous for theinductor coupled to the negative terminal of the array of speakers tohave an inductance that is substantially similar to the inductance ofthe inductor coupled to the positive terminal of the array of speakers,for example within 10 H or 10 μH.

It should be apparent to those skilled in the art that manymodifications besides those already described are possible withoutdeparting from the inventive concepts herein. The inventive subjectmatter, therefore, is not to be restricted except in the spirit of theappended claims. Moreover, in interpreting both the specification andthe claims, all terms should be interpreted in the broadest possiblemanner consistent with the context. In particular, the terms “comprises”and “comprising” should be interpreted as referring to elements,components, or steps in a non-exclusive manner, indicating that thereferenced elements, components, or steps may be present, or utilized,or combined with other elements, components, or steps that are notexpressly referenced. Where the specification claims refers to at leastone of something selected from the group consisting of A, B, C . . . andN, the text should be interpreted as requiring only one element from thegroup, not A plus N, or B plus N, etc.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a circuit for improving efficiency of an array of speakersthat includes an inductor and a capacitor.

FIG. 2 shows a circuit for improving efficiency of an array of speakersthat includes two inductors.

FIG. 3 shows a circuit for improving efficiency of an array of speakersthat includes two inductors, a resistor, and a capacitor.

FIG. 4 shows a circuit for improving efficiency of an array of speakersthat includes two inductors, a resistor, a capacitor, and threeswitches.

FIG. 5 is an abstract depiction of a box that can contain a circuit forimproving the efficiency of an array of speakers.

FIGS. 6 a and 6 b illustrate possible configurations for a specializedinductor that can be used in the circuit.

FIGS. 7 a and 7 b shows both theoretical and actual gain characteristicsof an embodiment of the circuit using the specialized inductor of FIG.6.

FIG. 8 shows a depiction of an original analog signal, a quantizedversion, and a reconstructed version of that signal created using thecircuit.

DETAILED DESCRIPTION

Various objects, features, aspects and advantages of the inventivesubject matter will become more apparent from the following detaileddescription of preferred embodiments, along with the accompanyingdrawing figures in which like numerals represent like components.

The following discussion provides many example embodiments of theinventive subject matter. Although each embodiment represents a singlecombination of inventive elements, the inventive subject matter isconsidered to include all possible combinations of the disclosedelements. Thus if one embodiment comprises elements A, B, and C, and asecond embodiment comprises elements B and D, then the inventive subjectmatter is also considered to include other remaining combinations of A,B, C, or D, even if not explicitly disclosed.

In some embodiments, the numbers expressing quantities of ingredients,properties such as concentration, reaction conditions, and so forth,used to describe and claim certain embodiments of the invention are tobe understood as being modified in some instances by the term “about.”Accordingly, in some embodiments, the numerical parameters set forth inthe written description and attached claims are approximations that canvary depending upon the desired properties sought to be obtained by aparticular embodiment. In some embodiments, the numerical parametersshould be construed in light of the number of reported significantdigits and by applying ordinary rounding techniques. Notwithstandingthat the numerical ranges and parameters setting forth the broad scopeof some embodiments of the invention are approximations, the numericalvalues set forth in the specific examples are reported as precisely aspracticable. The numerical values presented in some embodiments of theinvention may contain certain errors necessarily resulting from thestandard deviation found in their respective testing measurements.

As used in the description herein and throughout the claims that follow,the meaning of “a,” “an,” and “the” includes plural reference unless thecontext clearly dictates otherwise. Also, as used in the descriptionherein, the meaning of “in” includes “in” and “on” unless the contextclearly dictates otherwise.

The recitation of ranges of values herein is merely intended to serve asa shorthand method of referring individually to each separate valuefalling within the range. Unless otherwise indicated herein, eachindividual value is incorporated into the specification as if it wereindividually recited herein. All methods described herein can beperformed in any suitable order unless otherwise indicated herein orotherwise clearly contradicted by context. The use of any and allexamples, or exemplary language (e.g. “such as”) provided with respectto certain embodiments herein is intended merely to better illuminatethe invention and does not pose a limitation on the scope of theinvention otherwise claimed. No language in the specification should beconstrued as indicating any non-claimed element essential to thepractice of the invention.

Groupings of alternative elements or embodiments of the inventiondisclosed herein are not to be construed as limitations. Each groupmember can be referred to and claimed individually or in any combinationwith other members of the group or other elements found herein. One ormore members of a group can be included in, or deleted from, a group forreasons of convenience and/or patentability. When any such inclusion ordeletion occurs, the specification is herein deemed to contain the groupas modified thus fulfilling the written description of all Markushgroups used in the appended claims.

As used herein, and unless the context dictates otherwise, the term“coupled to” is intended to include both direct coupling (in which twoelements that are coupled to each other contact each other) and indirectcoupling (in which at least one additional element is located betweenthe two elements). Therefore, the terms “coupled to” and “coupled with”are used synonymously.

As used herein, a “capacitor” can be either a single component having acapacitance, or any combination of commonly used circuit components thathave the same Thevenin equivalent capacitance. As used herein, an“inductor” can be either a single component having an inductance, or anycombination of commonly used circuit components that have the sameThevenin equivalent inductance.

The inventive subject matter described herein is directed to a circuitthat minimizes signal reflection by substantially matching theimpedances of an audio signal source (e.g., a music player, anamplifier, etc.) and an array of speakers. As defined herein the term“array of speakers” means one or more speakers that are coupled witheach other in series and/or in parallel. In addition, some embodimentsof the circuit modify an audio signal to reduce quantization errors.

When a power source, such as an audio amplifier, feeds an array ofspeakers, the power transferred between the two is limited by the extentto which the impedances of the two are matched. Two types of impedancematching exist in electrical circuits. The first is maximum powertransfer matching and the second is electrical matching. Both aredescribed in detail below.

To obtain maximum power transfer from a source to a load, the loadimpedance must be matched to the source impedance. For a resistivecircuit, this match is achieved by ensuring the source and loadresistances are equal. For a complex circuit that includes a timedependent signals, such a match is achieved when the load impedance isthe complex conjugate of the source impedance.

Maximum power transfer matching is advantageous when the goal is to getas much power as possible out of a source and neither the source nor thefeed impedances can be changed. However, a system designed for maximumpower transfer is not always the most efficient system and, in fact,efficiency of such a system is half that of a system matched forreflectionless signal transmission.

When an electrical signal encounters a resistance (or an impedance), aportion of that signal can be reflected back toward the source. Inreflectionless matching, source and load impedances are matched suchthat substantially no part of an electrical signal is reflected back tothe source. In the case of a circuit dealing with a non-time dependentsignal, reflectionless matching is achieved by the extent to which thereal component of the source and load impedances are equal. For complexcircuits that handle AC signals, reflectionless matching is achievedwhen both the real and imaginary parts of the source and load impedancesare equal. Note that reflectionless matching and maximum power transfermatching have the same result for non-time dependent signals since asignal that is not time dependent will have no complex component.

Signal reflection is undesirable in many applications, including forexample, sound reproduction. Signal reflection can cause issuesincluding amplification of some frequencies, cancellation of otherfrequencies, and it can affect dynamic ranges and cause distortion.

Impedance matching in the case of a DC circuit (i.e., a circuit withouta complex source) can be achieved by matching a source resistance with aload resistance. As used herein, a “source resistance” is a Theveninequivalent resistance of a signal source while a “load resistance” is aThevenin equivalent resistance of a load. Matching these resistancesresults in maximum power dissipated across the load resistance. This canbe explained using Ohm's law and the equation describing electronicpower. Ohm's law is,V=IR

and electronic power is described as,P=IV

where V is voltage, I is current, R is resistance, and P is power.

For example, given a source having a constant resistance of 50Ω andconstant voltage of 100 V, Table 1 shows how power dissipated by theload is affected when the load resistance (RLoad) is varied from 45 to55Ω. Notably, power dissipation in the load is maximized when the sourceresistance is equal to the load resistance.

TABLE 1 V_(Load) I R_(Load) P_(Load) 47.4 1.05 45 49.861 47.9 1.04 4649.913 48.5 1.03 47 49.952 49.0 1.02 48 49.979 49.5 1.01 49 49.995 50.01.00 50 50.000 50.5 0.99 51 49.995 51.0 0.98 52 49.981 51.5 0.97 5349.958 51.9 0.96 54 49.926 52.4 0.95 55 49.887

This is also true for AC signal sources (e.g., an audio signal source).In AC circuits, there are two types of impedance matching:reflectionless matching and maximum power transfer matching. The circuitof the inventive subject matter represents an effort to achievereflectionless impedance matching between an audio signal source and anarray of speakers. For purposes of explanation of the concept, the audiosignal source is referred to as having a source impedance and the arrayof speakers is referred to as having a load impedance. It should berecognized that perfectly reflectionless matching may be unattainablefor audio signal sources and speaker arrays that have differentimpedances, but the inventive subject matter is drawn toward a novel newsystem of minimizing, or at least substantially reducing, signalreflection.

Impedance is different from resistance in that impedance takes intoaccount reactance as well as resistance. In a broad sense, impedance isa measure of opposition that a circuit presents to a current when avoltage is applied. More specifically, impedance has a resistance valueand a reactance value. It is expressed as,Z=R+jX

where Z is impedance, R is resistance, X is reactance, and j is theimaginary unit. Simply put, impedance can be any combination ofresistors, capacitors, and inductors, where the real component ofimpedance is resistance and the imaginary component is reactance (e.g.,capacitor and/or inductor reactance). Finally, impedance can similarlybe applied to Ohm's law as,V=IZ

thus extending Ohm's law to AC circuits.

Because of the similarities between DC circuits and AC circuits,impedance matching can be accomplished in AC circuits in a mannersimilar to resistance matching in DC circuits. As mentioned above, thereare two types of impedance matching that can be accomplished in ACcircuits. This results from the fact that impedance in AC circuits has acomplex reactance component as well as a resistive component. The firsttype of impedance matching results in maximum power transfer and isachieved by matching a source impedance with the complex conjugate of aload impedance. This is expressed as,Z _(S) =Z _(L)*

where Z_(S) is a source impedance and Z_(L) is a load impedance, theasterisk represents the complex conjugate of the variable Z_(L)*.

The second type of impedance matching results in minimized signalreflection from the load, Z_(L). To minimize signal reflection sourceimpedance and load impedance must be equal. This is expressed as,Z _(S) =Z _(L)

where Z_(S) is a source impedance and Z_(L) is a load impedance.

In the inventive subject matter, the goal is to minimize signalreflection between a load (e.g., an array of speakers) and a source(e.g., an audio signal source). In the case of an audio signal sourceand an array of speakers, both source and load impedances are functionsof frequency. Frequency dependence of reactance makes impedance matchingdifficult when frequency constantly varies as with an audio signal(e.g., music and/or audio books). The impedance of a source and a loadmay not vary by the same amount across all frequencies resulting in amismatch leading to signal reflection.

Embodiments of the inventive subject matter solve this problem byproviding a circuit comprising at least one inductor between the signalsource and the load. When an inductor having an appropriate inductanceis placed between an audio signal source and an array of speakers, itwill naturally provide some degree of impedance matching. An inductorcan be placed in different locations of the circuit (e.g., on thepositive side that couples the positive terminal of an audio signalsource to the positive terminal of an array of speakers and/or on thenegative side that couples the negative terminal of an array of speakersto the negative terminal of an audio signal source) to provide optimalimpedance matching for different speaker set ups and/or different rangesof audio signals.

The circuit additionally reduces quantization error instantiated duringanalog-to-digital conversion of an audio signal into a digital format.Quantization distortion is also known as quantization error, and itrepresents a difference between an actual analog value and acorresponding quantized digital value. Such errors are caused byrounding or truncation of an audio signal when it is encoded into adigital format.

Quantization errors are often manifested during signal reconstructionfrom a digital format: the signal is rebuilt into as many segments aswere originally sampled, resulting in digital “jumps” in signalamplitude along sloped portions. Since these jumps represent sharp risesor falls in a signal, the frequency associated with a jump is typicallyvery high (e.g., greater than 20 kHz).

Circuits of the inventive subject matter accomplish this by virtue ofthe reactance of inductors. Inductor reactance is expressed as,X _(L) =ωLwhere ω is frequency in radians and L is inductance. Therefore, as ωapproaches infinity, the reactance of the inductor similarly approachesinfinity. Based on the inductance of the particular inductor used insome embodiments of the circuit, reactance can approach infinity atdifferent rates. Thus, very high frequency signals will be excluded bythe inductors to some degree.

A simple analogy to mechanical systems provides an excellent example foreasy understanding. Inductors can be viewed as operating as a mass doesin a mechanical system. So for example, in a rotational system,flywheels are used to prevent sudden changes in rotational velocity. Thesame is true of inductors in an electrical system—inductors can beviewed as acting as flywheels to prevent sudden changes in current.Depending on the mass of the flywheel (i.e., inductance), sensitivity tochange can be tuned to a desired level.

The result of passing an audio signal through the circuit is thatquantization errors in the audio signal will be reduced according to theselected inductance for each inductor in the circuit (e.g., 10 μH, 15μH, 20 μH, 25 μH, 30 μH, 45 μH, 50 μH, 10-15 μH, 15-20 μH, 20-25 μH,25-30 μH, 30-35, μH, 35-40 μH, 40-45 μH, 45-50 μH, 50-100 μH, 100-200μH, 500 μH, 1-10 mH, 10-50 mH, 50-100 mH, 100-250 mH, 250-400 mH 400-500mH). Ultimately, the circuit is configured to not only match theimpedances of an audio signal source and an array of speakers, it isalso configured to modify an audio signal so that the signal betterimitates the original analog waveform.

FIG. 1 depicts an example circuit 100 disposed to couple an audio source(not shown) and an array of speakers 114 and reduces signal reflectionby improving impedance matching. The circuit 100 comprises an inductor110, a resistor 112, and a capacitor 116. The circuit 100 has fourterminals 102, 104, 106, and 108, coupled to a positive terminal of anaudio signal source, a negative terminal of an audio signal source, apositive terminal of an array of speakers, and a negative terminal of anarray of speakers, respectively. The audio signal source coupled toterminal 102, generates an audio signal that can fluctuate in frequency(e.g., primarily frequencies between 20 Hz and 20 kHz for soundsdetectable by most humans, but could also include frequencies between 20kHz-100 kHz and/or 100 kHz-200 kHz caused by noise in the signal). Theaudio signal generated by the audio signal source enters the circuit 100via terminal 102, passes to the array of speakers 114 via terminal 106,passes to the circuit 100 from the array of speakers 114 via terminal108, and finally passes back to the audio signal source (or to someother component, like ground) via terminal 104. In this particularconfiguration, the inductor 110 is located on the positive side of thecircuit 100 that connects directly to the positive terminals 102 and106. As used herein with respect to circuits, the term “directly” meansthere is no intervening electrical component.

The inductor 110 is generally configured to match the impedances of theaudio signal source and the array of speakers 114. In this embodiment,the audio signal source and the array of speakers 114 can each havedifferent Thevenin equivalent impedances, and the circuit 100 provides abalancing effect between the two to create an impedance match. Asmentioned above, the frequency of an audio signal can fluctuate, whichaffects the impedances of both the audio signal source and the array ofspeakers 114. As such, the circuit 100 substantially compensates forchanges in frequency to maintain impedance matching between the audiosignal source and the array of speakers. For example, if the audiosignal source has an impedance of 6 Ohms at some frequency, and thearray of speakers 114 has an impedance of 8 Ohms at that frequency, thenthe circuit will provide approximately 2 Ohms of impedance at thatfrequency. The result is that the audio signal source impedance combinedwith the inductor impedance is approximately 8 Ohms, which matches theimpedance of the array of speakers 114.

Similarly, if the audio signal source has a Thevenin equivalentimpedance of 8 Ohms at some frequency and the array of speakers 114 hasa Thevenin equivalent impedance of 6 Ohms at that frequency, the circuitwill again provide 2 Ohms of impedance at that frequency. This helps tobalance the two because when looking from the audio signal source towardthe array of speakers, the equivalent impedance of the audio signalsource with the circuit 100 is 8 Ohms, thus eliminating, orsubstantially reducing, reflection of the audio signal from the array ofspeakers back towards terminal 102.

Since the frequency of the audio signal tends to vary over time, theimpedance selected for inductor 110 is generally determined about anaverage frequency over the span of a selection of music, or a preferredfrequency, such as a sustained note. Where inductor 110 is a variableinductor, a control unit (not shown) could be included that allows auser to select the inductance of inductor 110. In some embodiments wherethe inductance of the audio source and the array of speakers 114 isknown, a control unit (not shown) of inductor 110 could allow a user toselect a frequency, and inductor 110 will adjust to that frequency. Forexample, the control unit could be configured to impedance match thebest at 1 kHz, at 5 kHz, and at 10 kHz, where a user simply selects thepreferred frequency of the circuit. Where inductor 110 is a variableinductor, a user could vary the inductance of inductor 110 until thecircuit matches the impedance of the input audio source and speakerarray 114 about the preferred frequency, or preferred frequencies wherethe user wishes to have one preferred frequency for one song, andanother preferred frequency for a second song.

The circuit 100 also includes a resistor 112 and a capacitor 116 thatare in series with each other while in parallel with the array ofspeakers 114. The resistor 112 and capacitor 116 act as a high-passfilter preventing high frequency signals above a certain frequency fromreaching the array of speakers 114. This is caused by the nature of theimpedance of a capacitor, described as,

$X_{C} = \frac{1}{\omega\; C}$

where X_(C) is the impedance of a capacitor, ω is the frequency of thesignal, and C is capacitance. As frequency approaches infinity, theimpedance of a capacitor approaches 0. The effect of this is that asignal having a very high frequency will tend to pass through resistor112 and capacitor 116 instead of through the array of speakers 114,since current tends to travel through the path of least resistance (orimpedance). By placing a resistor 112 and a capacitor 116 in series witheach other, a “drop-off” in signal occurs above some frequency. Changingthe resistance and capacitance will change both the slope of the rampportion of the drop-off and the frequency above which the drop-offoccurs.

Resistance in embodiments of the circuit 100 can be varied in a numberof different ways. In one way, a resistor can be incorporated into thecircuit 100 such that it is simply plugged in, and when a differentresistance is desired, a different resistor can be plugged in. Anotherway to vary resistance is to incorporate a potentiometer into thecircuit. Potentiometers are typically manually operated, althoughelectronically operated variable resistors are also contemplated.Capacitor control can be achieved similarly. For example: capacitorshaving difference capacitances can be plugged into the circuit dependingon the desired traits of the circuit; variation in capacitance can beachieved mechanically via manual manipulation (e.g., operating a screw);or capacitance can be modified electronically using, for example, acomputer or a microcontroller. Similar to the exemplary control unitabove, another control unit could be included that allows a user toselect the resistance of resistor 112 and capacitance of capacitor 116,or the drop-off frequency itself.

FIG. 2 depicts another exemplary circuit 200 for matching impedancebetween an audio source and an array of speakers 214. Instead of havingjust one inductor as shown in the circuit 100 of FIG. 1, the circuit 200includes two inductors 210 and 212. As shown, the two inductors 210 and212 are located on different sides of the circuit 200. Specifically, afirst inductor 210 is located on the positive side of the circuit 200and is coupled to the positive terminal of the audio signal source andthe positive terminal of the array of speakers. A second inductor 212 islocated on the negative side of the circuit 200 and is coupled tonegative terminal of the audio signal source and the negative terminalof the array of speakers. FIG. 2 also shows that the resistor andcapacitor of FIG. 1 can be optional to the circuit.

Providing a second inductor 212 on the other side of the circuit 200(i.e., between the negative terminal of the array of speakers 208 andthe negative terminal of the audio signal source 204) further helps tomatch impedance. An audio signal is typically an AC sinusoidal signaland thus fluctuates about a 0 value (i.e., 0 volts). Thus, the audiosignal drives current in both directions through the circuit 200, whichcauses the diaphragm(s) of the speaker(s) in the array of speakers to beactively pulled in different directions by the voice coil(s) to createsound waves. Because current travels in both directions through thecircuit and the array of speakers, it can be advantageous for thecircuit to be symmetrical. This symmetry results in the ability of thecircuit to compensate for the electromagnetic effects caused by movementof the permanent magnet in the speaker driver(s) relative to the voicecoil of the driver(s). Having inductors on both sides allows the circuit200 to absorb current created by such movement and allow the speakerdiaphragm(s) in the array of speakers 214 to more freely resonatewithout experiencing as much mechanical reluctance caused by theelectromagnetic effect of the permanent magnet(s) moving relative to thevoice coil(s).

FIG. 3 is a circuit diagram of an embodiment of the circuit 300 havingtwo inductors 310 and 312, a resistor 316, and a capacitor 318. Asdiscussed in relation to FIG. 1, the resistor 316 and capacitor 318 actas a low-pass filter, and, as discussed above, the inductors 310, 312help to achieve substantially reflectionless impedance matching betweenthe audio signal source (e.g., an amplifier) and the array of speakers314. An audio signal enters the circuit through terminal 302, passesthrough inductor 310, and is affected by the filtering properties of theresistor 316 and the capacitor 318 before leaving the circuit viaterminal 306. Terminal 306 is coupled to the positive terminal of thearray of speakers 314, which the signal passes through before returningto the circuit via terminal 308. From there, it passes back throughterminal 304 to the audio signal source.

The embodiment of FIG. 3 brings together two key advantageous aspects(both described in more detail above): (1) the advantages of a circuit300 having an inductor 310 between the positive terminals of the audiosignal source and the array of speakers, as well as an inductor 312between the negative terminals of the same, and (2) the advantages ofhaving a resistor 316 and capacitor 318 across the positive and negativeterminals of the array of speakers 314.

Having the resistor 316 and the capacitor 318 coupled to the positiveand negative terminals allows the signal or a portion of the signal topass back to the audio signal source without first travelling throughthe array of speakers 314, as discussed above. In this embodiment, anysignal, or portion of a signal, circumventing the array of speakers 314will nevertheless encounter this inductor 312.

When signal passes through the resistor 316 and the capacitor 318, andsubsequently encounters inductor 312, the signal that passed through thespeakers is phase shifted relative to the signal that has passed throughthe resistor 316 and the capacitor 318. Signal that has passed throughthe resistor and the capacitor encounters signal that has passed throughthe array of speakers and cancels out the phase shift brought on by thearray of speakers, thus allowing the inductor 312 to operate moreeffectively and in sync with the other inductor 310. One advantageousresult of this, as discussed above, is that the speakers are able tomore freely mechanically resonate since the inductors are able to absorbelectromagnetically induced current.

FIG. 4 is a circuit diagram of another embodiment of the circuit 400. Itis similar to the circuit of FIG. 3, except that it includes switches.The switches 420, 422, 424 can be closed to essentially take thecorresponding circuit component out of the circuit. For example: ifswitch 420 is closed, then inductor 410 is shorted out of the circuit;if switch 422 is closed, then resistor 416 and capacitor 418 are removedfrom the circuit; and if switch 424 is closed, then inductor 412 isshorted out of the circuit.

Operating switches 420, 422, and 424 in a coordinated fashion canproduce desirable results, such as phase shifting of an audio signal.The switches 420, 422, and 424 can be mechanical relays, electronicswitches such as, for example, transistors, or any other switches knownin the art. Switches 420, 422, and 424 can additionally be operatedeither mechanically or electronically. Mechanical operation involvesmechanically changing the state of a switch, while electronic operationinvolves changing the state of a switch using electricity (e.g., atransistor).

A phase shift in this context can refer to any change in phase of asignal, or in the phase difference between two or more signals. In theembodiment of FIG. 4, the desired phase shift involves shifting an audiosignal in the time domain. In other words, the signal is delayed by somefraction of time, causing the wave form to lag in time behind where itmight otherwise be. This effect is desirable for the same reason asdescribed above with regard to canceling phase shifting. By providing anavenue to control phase shifting, the negative effects of phase shiftingcaused by the array of speakers 414 can be minimized. Thus, phaseshifting can be used to advantageously to enable the speaker(s) of thearray of speakers 414 to resonate more freely, which allows the array ofspeakers to perform better across a broader range of frequencies than itotherwise would without the circuit 400.

FIG. 5 shows an embodiment of the inventive subject matter as a blackbox 500. The black box 500 can contain any embodiment of the circuitsincluding those described above. The black box 500 has four terminals502, 504, 516, and 518 that are used to transmit an audio signal.Terminal 502 is coupled to the positive terminal of an audio signalsource, and terminal 516 is coupled to a positive terminal of a speakeror array of speakers. Terminal 518 is coupled to a negative terminal ofa speaker or array of speakers. Finally, terminal 504 is coupled to thenegative terminal of an audio signal source. In alternative embodiments,terminal 502 and 504 could be incorporated into a single input port andterminal 516 and 518 could be incorporated into a single output portwithout departing from the scope of the current invention.

In some embodiments, the positive terminal of an audio signal sourcetransmits a signal to a speaker, while the negative terminal completes acircuit with the speaker and the audio signal source. In most hometheater systems, for example, the audio signal source is an amplifier.Other examples of signal sources include CD players, digital musicplayers (e.g., cell phones, tablets, iPod touches and similar devices,etc.). It is foreseeable that the device of some embodiments can be usedwith any music signal from any source, even if the source does notprovide a digitally reconstructed signal (e.g., a vinyl record player).Any speaker associated with an embodiment of the system has a positiveand negative terminal corresponding to the positive and negativeterminals of the audio signal source. The black box 500 is a unitdesigned to sit between the audio signal source and the array ofspeakers, thus it has four terminals 502, 504, 516, and 518.

The black box 500 can additionally have a number of inputs 506, 508,510, 512, and 514. Inputs 506, 508, and 510 provide for control ofswitches that can optionally be included in the circuit contained withinthe black box. First switch control 506 provides for control of a firstswitch, for example switch 420, second switch control 508 provides forcontrol of a second switch, for example switch 422, and third switchcontrol 510 provides for control of a third switch, for example switch424. Additional switches could be incorporated without departing fromthe scope of the current invention. Switching inputs are optional,however, and depend on the configuration of the circuit within the blackbox 500. Inputs 512 and 514 provide for control of a resistor andcapacitor, respectively. Resistor control 512 allows for variation of aresistor in a circuit contained with the black box 500, while capacitorcontrol 514 allows for variation of the capacitance of a capacitor. Thisfeature is optional, and inclusion in the device depends on whether avariable resistor and/or capacitor has been used in the circuit.

The switches could be manual switches controlled by moving a switch or abar across an interface, or could be electronic switches that arecontrolled by a centralized control interface (not shown). Inembodiments where the switches are manual switches, indicators arepreferably placed near the switches to indicate how the properties ofthe circuit are changed. For example in one embodiment switch control506 could have an indicator showing that it controls inductor 1, whichcould be turned ON for impedance matching or OFF to deactivate impedancematching, and/or could even have indicators that show that a firstposition sets inductor 1 to a first inductance of 25 μH, a secondposition that sets inductor 1 to a second inductance of 50 μH, and athird position that sets inductor 1 to a third inductance of 100 μH.Similar indicators could be provided to adjust the resistance and/orcapacitance of an RC circuit, and/or another inductor included in blackbox 500. Where the switches are electronic inputs, a separate userinterface (not shown) would preferably have such indicators.

FIGS. 6 a and 6 b show exemplary inductors 600 a, 600 b that can be usedwith any of the embodiments from FIGS. 1-5 to produce better impedancematching over a range of frequencies. Inductors have three maincomponents: (1) a coiled wire 602 a, 602 b; (2) a core 604 a, 604 b; and(3) a sometimes one or more gaps 606 a, 606 b. When current passesthrough the coiled wire 602 a, 602 b, a magnetic field is generatedwithin the core 604 a, 604 b and gap 606 a, 606 b. Depending on thematerial within the core 604 a, 604 b and the gap 606 a, 606 b, themagnetic field can have different effects.

Providing a component having an impedance between two other componentshaving two different resulting impedances normally results in animpedance match only in the vicinity of a specific signal frequency.This results because impedance is a function of frequency, and thus asfrequency changes, impedances change at different rates. Embodiments ofthe inventive subject matter solve this problem by using inductors whoseimpedances vary at a rate that is proportional to the rate of change ofboth the impedance of the audio signal source and the impedance of thearray of speakers. In this way, the impedances remain matched across awide range of signal frequencies.

In essence, the ability of the inductor to vary at a desired rate makesmatching across a range of frequencies possible. For example, if theaudio signal source has an impedance of 2 Ohms at 100 Hz and the arrayof speakers has an impedance of 8 Ohms at 100 Hz, then the inductor willhave approximately 6 Ohms impedance at 100 Hz to create an impedancematch. If those same components have impedances of 6 Ohms and 24 Ohms at10 kHz, respectively, then the inductor will ideally have an impedanceof approximately 18 Ohms at 10 kHz to create an impedance match.

Inductance can be described generally as,

$L = \frac{\mu\; N^{2}A}{l}$

where μ is the permeability of the core, N is the number of times thewire has been wound around the core, A is the cross-sectional area ofthe core, 1 is the length of the core, and L is inductance. Thus,altering permeability of the core affects inductance and thus theresponse characteristics of an inductor. Core material can also affectthe resonant characteristics of an inductor, because a core material hasa characteristic frequency at which the inductor exhibits the highestinductance. As a result, one or more core materials can be selected suchthat the inductor's performance peaks at particular frequency or acrossa range of frequencies.

The concept of selecting a core material to provide peak performance ata particular frequency can be expanded by creating an inductor usingmultiple materials having multiple characteristic frequencies. Forexample, a part of the core can be made from one material having apermeability and characteristic frequency, while the rest of the corecan be made from another material having a different permeability andcharacteristic frequency. Such a combination results in the inductorhaving two different performance peaks at two different frequencies. Forapplications such as audio signals, the overall effect is that suchinductors are able to perform better over a broader range of audiosignal frequencies. For example, an inductor can one or more corematerials (e.g., blended or having boundary layers) where the materialor materials all have different material properties (e.g., differentpermeabilities).

By carefully selecting core materials based on desired permeability andmaterial properties, an inductor can be created that exhibitsadvantageous characteristics across a broader range of frequencies thana standard inductors having only a single core material.

The same concept can be applied the gap portion of the inductor. It canbe made up of one or more materials selected based on desiredpermeability and desired effects across a range of frequencies.

The materials used in the core and gap can be some combination ofparamagnetic, ferromagnetic, and/or diamagnetic materials. Someparamagnetic materials include (written in the form of, “material(approximate relative permeability)”): air (1.0000004), aluminum(1.00002), and palladium (1.0008). Some ferromagnetic materials include:2-81 Permalloy powder (130), cobalt (250), nickel (600), ferroxcube 3(1,500), mild steel (2,000), iron (5,000), silicon iron (7,000), 78Permalloy (100,000), mumetal (100,000), purified iron (200,000), andsuperalloy (1,000,000). Some diamagnetic materials include: bismuth(0.99983), silver (0.99993), lead (0.99993), copper (0.999991), andwater (0.999991).

The inductor can be made in many configurations based on the frequencyrequirements. For example, the frequency range that the inductor is tobe used with is an important factor to consider when determining thesize of the gap portion of the inductor. The gap and/or can include, forexample, Ti02-6 (e.g., 30-35%, 35-40%, 40-45%, 45-50%, 50-55%, 55-60%,60-65%, 65-70%, 70-75%, 75-80%, 80-85%, 85-90%, or 90-98% by volume),aluminum (e.g., 5-10%, 10-15%, 15-20%, 20-25%, 25-30% by volume), Cobalt(e.g., 2-5%, 5-10%, 10-15%, 15-20%, 20-25%, 25-30% by volume), andsometimes tin binders (0.5-2%, 2-4%, 4-6%, 6-8%, 8-10%). The proportionsfor each material used in the core and/or gap are determined based on afrequency range to be used with the inductor. In addition to thespecific materials listed above, it is contemplated that these rangescan similarly apply to any paramagnetic, diamagnetic, or ferromagneticmaterial that is used to create the inductor.

Any combination of materials is contemplated to alter the properties ofinductor 600 a or inductor 600 b. For example, 604 a could comprise aparamagnetic material comprising at least 60% by volume while gap 606 acould comprise a ferromagnetic material comprising at least 30% byvolume, or 604 b could comprise a ferromagnetic material comprising atleast 70% by volume while gap 606 b could comprise a diamagneticmaterial comprising at least 25% by volume. A plurality of gaps (notshown) could be used to further vary the properties of the inductor,such as an inductor having a paramagnetic core, a first gap offerromagnetic material, and a second gap of diamagnetic material. One,two, three, or more such gaps could be embedded in an inductor to alterits properties. In other embodiments, the core itself is partitionedinto segments of different materials, such as a first segment ofparamagnetic material, a second segment of diamagnetic material, a thirdsegment of ferromagnetic material, and a fourth segment of paramagneticmaterial.

FIGS. 7 a-7 b show a number of graphs 700 a, 700 b depicting gaincharacteristics of systems using embodiments of the circuit withinductors from FIG. 6. Graph 700 a shows simulated gain relative to 1kHz both with and without an embodiment of the circuit activated tomodify the signal. Data set 702 a depicts a simulation of gain relativeto 1 kHz without an embodiment of the circuit connected between theaudio source and the speaker array, and data set 704 a depicts asimulation of gain relative to 1 kHz with an embodiment of the circuitconnected between to the audio source and the speaker array. Thesimulated results show that passing a signal through a system using anembodiment of the circuit should result in a drop in gain relative to asystem that is not using an embodiment of the circuit beginning around 2kHz.

Graph 700 b depicts actual measured results comparing the gain of asystem that does not use an embodiment of the circuit compared to asystem that does use an embodiment of the circuit. Data set 702 bdepicts a simulation of gain relative to 1 kHz without an embodiment ofthe circuit, and data set 704 b depicts a simulation of gain relative to1 kHz with an embodiment of the circuit connected. The gain fluctuatesmore in the real-world system than in the simulated system, due in largepart to un-modeled nonlinear behaviors in many of the components of thesystem. However, the real-world results also show that passing a signalthrough a system using an embodiment of the circuit should result in adrop in gain relative to a system that is not using an embodiment of thecircuit beginning around 2 kHz.

FIG. 8 shows a depiction of an example audio signal in three differentforms. The first graph 800 shows an original audio signal 801 before ithas been converted into a digital format, the second graph 820 shows theaudio signal 803 after it has been reconstructed after digitization, andthe third graph 840 shows the signal 805 after it has passed through anembodiment of the inventive subject matter. The original audio signal801 is first divided into different segments of time, which correspondto sample times 802. At each sample time 802, the amplitude of thesignal is detected and stored for an entire time segment. For example,when the original audio signal 801 is sampled at time 804, the amplitudeof the signal at that time is extended for the duration of that timesegment. Thus, segment 806 becomes segment 808 as seen in graph 820.Finally, after passing through an embodiment of the circuit, signalsegment 808 appears approximately as signal segment 810 in graph 840,which more closely resembles the analog input than digital output 808.

It should be apparent to those skilled in the art that manymodifications besides those already described are possible withoutdeparting from the inventive concepts herein. The inventive subjectmatter, therefore, is not to be restricted except in the spirit of theappended claims. Moreover, in interpreting both the specification andthe claims, all terms should be interpreted in the broadest possiblemanner consistent with the context. In particular, the terms “comprises”and “comprising” should be interpreted as referring to elements,components, or steps in a non-exclusive manner, indicating that thereferenced elements, components, or steps may be present, or utilized,or combined with other elements, components, or steps that are notexpressly referenced. Where the specification claims refers to at leastone of something selected from the group consisting of A, B, C . . . andN, the text should be interpreted as requiring only one element from thegroup, not A plus N, or B plus N, etc.

What is claimed is:
 1. A circuit for improving efficiency of an array ofspeakers having a positive and negative terminal, comprising: a firstcircuitry configured to couple the positive terminal of the array ofspeakers with an audio signal source, wherein the first circuitrycomprises an inductor configured to reduce reflection of an audio signalby the array of speakers; and a second circuitry configured tooperatively interpose a capacitor and an electrically variable resistorbetween the positive terminal of the array of speakers and the negativeterminal of the array of speakers.
 2. The circuit of claim 1, whereinthe inductor comprises a paramagnetic material.
 3. The circuit of claim1, wherein the inductor comprises a ferromagnetic material.
 4. Thecircuit of claim 1, wherein the variable resistor comprises apotentiometer.
 5. The circuit of claim 1, wherein the inductor has a gapthat comprises at least one of a ferromagnetic and diamagnetic material.6. The circuit of claim 1, wherein the variable resistor is controlledby a computer programmable logic controller.
 7. The circuit of claim 1,wherein the capacitor is a variable capacitor.
 8. The circuit of claim7, wherein a capacitance of the variable capacitor is controlled by acomputer programmable logic controller.
 9. The circuit of claim 7,wherein the variable capacitor is a digitally tuned capacitor.
 10. Thecircuit of claim 1, further comprising a third circuitry coupled to thenegative terminal of the array of speakers, and further coupled to theaudio signal source, wherein the third circuitry comprises a secondinductor.
 11. The circuit of claim 10, wherein the third circuitrycomprises a switch in parallel with the second inductor.
 12. The circuitof claim 1, wherein the first circuitry comprises a switch in parallelwith the inductor.
 13. The circuit of claim 1, wherein the secondcircuitry comprises a switch in series with the resistor and thecapacitor.
 14. An audio enhancement kit comprising: a first circuitrycomprising a first inductor configured to be coupled to a positiveterminal of an array of speakers; a second circuitry comprising a secondinductor configured to be coupled to a negative terminal of the array ofspeakers; and a third circuitry that couples to both the positiveterminal and the negative terminal of the array of speakers, comprisinga resistor and a variable capacitor.
 15. The audio enhancement kit ofclaim 14, wherein the resistor comprises a variable resistor.
 16. Theaudio enhancement kit of claim 15, wherein the variable resistorcomprises an electronically variable resistor.
 17. The audio enhancementkit of claim 14, wherein the variable capacitor comprises anelectronically variable capacitor.
 18. A circuit for improvingefficiency of an array of speakers having a positive and negativeterminal, comprising: a first circuitry configured to couple thepositive terminal of the array of speakers with an audio signal source,wherein the first circuitry comprises an inductor configured to reducereflection of an audio signal by the array of speakers; and a secondcircuitry configured to operatively interpose a variable capacitor andan electrically varied variable resistor between the positive terminalof the array of speakers and the negative terminal of the array ofspeakers, wherein a capacitance of the variable capacitor is controlledby a computer programmable logic controller.
 19. The circuit of claim18, wherein the variable resistor comprises a potentiometer.
 20. Thecircuit of claim 18, further comprising a third circuitry coupled to thenegative terminal of the array of speakers, and further coupled to theaudio signal source, wherein the third circuitry comprises a secondinductor.
 21. The circuit of claim 20, wherein the third circuitrycomprises a switch in parallel with the second inductor.
 22. The circuitof claim 18, wherein the first circuitry comprises a switch in parallelwith the inductor.
 23. The circuit of claim 18, wherein the secondcircuitry comprises a switch in series with the resistor and thecapacitor.
 24. A circuit for improving efficiency of an array ofspeakers having a positive and negative terminal, comprising: a firstcircuitry configured to couple the positive terminal of the array ofspeakers with an audio signal source, wherein the first circuitrycomprises an inductor configured to reduce reflection of an audio signalby the array of speakers; and a second circuitry configured tooperatively interpose a digitally tuned variable capacitor and anelectrically varied variable resistor between the positive terminal ofthe array of speakers and the negative terminal of the array ofspeakers.
 25. The circuit of claim 24, further comprising a thirdcircuitry coupled to the negative terminal of the array of speakers, andfurther coupled to the audio signal source, wherein the third circuitrycomprises a second inductor.
 26. The circuit of claim 25, wherein thethird circuitry comprises a switch in parallel with the second inductor.27. The circuit of claim 24, wherein the first circuitry comprises aswitch in parallel with the inductor.
 28. The circuit of claim 24,wherein the second circuitry comprises a switch in series with theresistor and the capacitor.